Projections #173

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gregwebs opened this Issue Oct 12, 2013 · 4 comments

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gregwebs commented Oct 12, 2013

My thought process now is that the user should declare the projections available.

Then we need an overloaded lens (mkFields from lens) to use a simple name instead of a projection-overloaded name. Alternatively we could wait until 7.10 or try to start using vinyl.

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gregwebs commented Oct 27, 2013

I looked into dependent-map, but that makes everything a Maybe. A DMap lets us have a map of field names to values, where unlike a normal Map the values can have different types corresponding to their keys.
The only problem with DMap is that to retrieve a field we now do a lookup that returns a Maybe. But we already know based on the query what fields are available, so this is a little unsatisfactory.

Groundhog returns tuples from projections, but the problem is there is no longer a corresponding field name. So you could have a projected tuple of (Text, Text, Text) that would be easy to screw up.

We could instead use an extensible records package such as vinyl. However, I don't know how we can give back a type for our projections. Instead all functions operating on projections would be duck-typed based on the field. This could work if we stick with labels that are record-prefixed as persistent does now. Vinyl adds a lens dependency and I believe it is required for writing duck-typed functions.
http://www.jonmsterling.com/posts/2013-04-06-vinyl-modern-records-for-haskell.html
A big downside of vinyl is that for a function using many record fields, the type signatures will start getting unwieldy. This is also true of using some kine of Has typeclass.

dependent-map has simple type signatures, integrates well with persistent, and does not add other dependencies. The price we pay is that every access is a Maybe.

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snoyberg commented Jul 19, 2015

Closing out old issues, please reopen if still relevant

@snoyberg snoyberg closed this Jul 19, 2015

This seems like it should still be relevant, can someone with the right permissions reopen it?

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gregwebs commented Oct 2, 2015

We prefer to use tithing issues for bugs or features that are being actively worked on

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